Method of gold plating

ABSTRACT

Method of gold plating particularly suited for the gold plating of localized conductive areas such as contacts on printed circuit boards, wherein the surface to be plated is first cleaned and activated without any current being applied by rubbing onto said surface a non-displacement type of gold electrolyte solution carried by absorbent material forming part of a gold applicator assembly that includes a gold anode. A plating current is then passed through said electrolyte solution during continued rubbing to effect the plating of gold on said surface as the cathode. The preferred electrolyte is a water solution of an alkali metal gold cyanide containing substantially no free cyanide or alkali metal cyanide in excess of the cyanide bound to the gold present in said water solution. The method is self-indicating in that so long as the plating maintains a superficial dark or brownish coloration throughout the plating cycle, the gold electrodeposit is of satisfactory quality and is firmly adherent, whereas if the gold electrodeposit exhibits a clear gold color, or if no plating takes place, that indicates a poor or broken plating connection in the plating circuit. Rubbing of the electrodeposit with the applicator during the plating operation, if there are poor or broken plating connections, quickly removes the superficial dark coloration of the electrodeposit and imparts thereto a clear gold color of substantially the same clarity of gold color as that of the gold applicator used. If this occurs, it is only necessary to correct the bad plating connection and continue plating.

United States Patent 11 1 Rapids METHOD OF GOLD PLATING [75] Inventor:Felix R. Rapids, Chicago, 111.

Assignee: Rapid Electroplating Process, Inc.,

Chicago, Ill.

22 Filed: Nov. 18, 1971 [21] Appl. No.: 200,045

Related US. Application Data [63] Continuation-impart of Ser. No. 2,982,Jan. 15, 1970,

abandoned. I

52] us. c1 ..204/15, 2 04/46, 204/224 [51] Int. Cl. C23b 5/48, C23b5/24, 1323p H02 [58] Field of Search 2'04/15, 224 R, 43, 204/44, 46

[56] References Cited I UNITED STATES PATENTS 1,552,591 9/1925 Batenburg20'4 224 R 1,787,431 1/1931 Batenburg... 204/224 R 2,061,591 11/1936Rapids-....- 204/224 R 2,958,928 ll/l960 Bain, Jr. et al. 204/152,812,299 ll/l957 Volk ..-'204/44 OTHER PUBLICATIONS ModernElectroplating, (1963) pp. 207-223 Primary ExaminerThomas Tufariello J.Arthur Gross et a].

2nd Edition, Lowenheim Aug. 28, 1973 [57] ABSTRACT Method of goldplating particularly suited for the gold plating of localized conductiveareas such'as contacts on printed circuit boards, wherein the surface tobe plated is first cleaned and activated without any current beingapplied by rubbing onto said surface a nondisplacement type of goldelectrolyte solution carried the gold electrodeposit is of satisfactoryquality and is firmly adherent, whereas if the gold electrodepositexhibits a clear gold color, or if no plating takes place,

that indicates a poor or broken plating connection in the platingcircuit. Rubbing of the electrodeposit with the applicator during theplating operation, if there are poor or broken plating connections,quickly removes the superficial dark coloration. of the electrodepositand imparts thereto a clear gold color of substantially the same clarityof gold color as that of the gold applicator used. If this occurs, it isonly necessary to correct the bad plating connection and continueplating.

6 Claims, 3 Drawing Figures o c Q 0 l3 I l /20 Patented Aug. 28, 19733,755,089

@ oogo D INVENTOR. F54 /x K. FA '5 M firrnm ,wh-rw l w a y ATTORNEYSMETHOD OF GOLD PLATING RELATED INVENTIONS This is a continuation-in-partof my pending applicationSer. No. 2,982, filed Jan. 15, 1970 and nowabandoned.

DESCRIPTION or THE PRIOR ART ried out without the use 'of plating baths,wherein a nondisplacement type alkali metal gold cyanide electrolyte isused that contains no free acid and no free alkali metal cyanide, and inaccordance with which gold electrodeposits are obtained which areself-indicating as to their satisfactory or unsatisfactory character fouse as electrical connections or contacts.

SUMMARY OF THE INVENTION The present invention provides an improved,rapid and, more convenient method for electrodepositing gold on surfaceareas of electrically conductive members to provide new contacts or torepair old contacts for use in an electrical circuit, such as a printedcircuit board having copper or other electrically'conductive members. it

The process requires no stationary plating'tank or. other stationaryequipment but involves manually rubbing the surface to beplated, bothbefore and during I plating, with an electrolyte applicator thatsuitably comprises an absorbent, inert material for carrying the goldelectrolyte disposed on or about a gold-surfaced anode of suitable formand design for manual handling. The absorbent material, which maybe afabric of natural fibers, such as cotton, or of inert artificial-fibers,such as polyacrylonitrile, is wetted with an electrolyte of my inventionparticularly adapted for gold plating. A non-corrosive electrolyte,incapable of displacement plating, is used, such as an aqueous solutionconsisting essentially of water and an alkali metal gold cyanide inwhich there is no substantial excess of any alkali metal cyanide orfreecyanide, over and beyond that bound to or held by the gold as a partof the gold cyanide moiety.

One of the imprtant uses of this plating method is the repair andmaintenance of gold platings on.electronic and other contact surfaces.This includes printed cir- .cuits with as many as two dozen or moreindividual The situation is further complicated by the fact that thistype of plating or replating must usuallybe done by operators withlittle or no knowledge of electroplating. No equipment or method otherthan through actual use has previously been available that wouldindicate the quality of the gold plating on each contact.

My present invention provides a gold plating electrolyte and methodthat:

' only one single, relatively harmless, non-corrosive solution that canbe used practically anywhere without danger to operator or to criticalequipment.

type that can be rubbed on areas to be plated, with a manual goldapplicator, without current, as the final cleaning and activatingoperation before plating to ensurefirm adhesion.

3. Is self-indicating in that the maintencance of a su perficial dark orbrownish coloration throughout the plating cycle establishes thatsatisfactory gold plating is being carried out, whereas no plating, or aclear gold color of plating on any contact or contacts, immediatelyindicates a bad or broken plating connection which must be corrected toensure good plating. The rubbing of the applicator over a gold platedsurface during the plating operation will remove any superficial darkcoloration and expose the underlying clear gold color of contacts orareas having bad'or broken platingconnections, but will not removesuch-superficial dark coloration-during plating if the plating circuitis in good operating condition.

4.. Ensures purity of the gold plating and permits the use of more rigidmetals, such asnickel, for the backing or reinforcing of puregold-surfaced anodes, because such more rigid metals will not codepositwith the gold from my solution operated under the conditions of mymethod.

5. My gold electrolyte'is a non-displacement type, consisting of sodiumor potassium gold cyanide or 'a combination of both sodium and potassiumgold cyanides in water with possibly a small amount of wetting agent.Commercial gold cyanide and acid goldplating solutions cannot be usedfor carrying out my method. This holds true for the gold platingelectrolyte of aforementioned Underwood patent.

6. My electrolyte solution when used with the gold applicator utilizescurrent densities of from 400 to 700 amperes or more/sq. ft. andrelatively high voltages of about 8 to 16 volts (direct current).

It is therefore an'object of this invention to provide an improvedmethod of gold plating to form gold contacts of highelectrical-conductivity and of optimum thickness, flexibility andadherence, wherein plating'is carried out to a point that isself-indicated by the appearance of the electrodeposited gold as havinga superficial dark or brownish coloration in striking contrast to theclear gold color exhibited by the goldsurfaced anode of the goldapplicator assembly.

It is a further important object of this invention to provide a reliablemethod of forming gold plating that provides good electrical connectionsand that can be distinguished from insufficiently plated contacts orareas by their dark appearance as contrasted with the clear gold eolorof the plating that indicates poor and- /or inadequate connections inthe plating, circuit.

Other and further important objects of the invention will becomeapparent from the following description of certain preferred embodimentstaken in conjunction with the accompanying drawing, although variationsRequires in order to obtain a good gold plating,

2. Employs a gold solution of a non-displacement and modifications maybe effected without departing from the spirit and scope of the novelconcepts of the disclosure, and in which: 7

FIG. 1 of the drawing illustrates somewhat schematically equipmentsuitable for carrying out my gold plating method as applied to the goldplating of contacts on a printed circuit board;

FIG. 2 is an enlarged fragmentary plan view of the conductive areas thathave been gold plated, showing the difference in appearance betweencontacts properly plated in accordance with my invention and others notproperly plated; and

FIG. 3 is a back view of a preferred shorting strip for use with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to prepare the surfacefor carrying out the method of my invention in the gold plating of anelectrical contact or other conductive surface, it is essential that thesurface be properly cleaned and otherwise prepared for receiving theelectrodeposition of gold over the localized areas that are to fonnelectrical connections or contacts in an electrical circuit. Preparationof the surface is accomplished prior to the application thereto of thegold electrolyte by scouring the surface with steel wool, such as 3/0 or4/0 steel wool, to thoroughly polish the same. Such steel wool iscommercially available. After polishing, the surface is wiped clean witha lint-free cloth. The surfaces to be plated, which may be copper,brass, or nickel, or'even ferrous metal surfaces, should be thoroughlypolished in this manner over the area to be plated prior to theapplication of any electrical plating current.

With the current still off, a film of my gold plating solution, orelectrolyte, is rubbed over the polished surface area by the use of thegold electrolyte applicator later to be described, indicated by thereference numeral 11. Such rubbing action activates'the surface forsubsequent electrodeposition of gold, without, however, anyobjectionable displacement plating taking place prior to energizing theplating circuit.

As shown on the drawings, the reference numeral indicates a tray, basinor other portable receptacle over which or in connection with which myplating method can suitably be carried out. The use of a plating tank,as such, is obviated entirely. The receptacle 10 serves as a portablereservoir for an electrolyte solution capable of depositing gold, and/orfor collecting any excess electrolyte during the application thereof toelectrically conductive material, but the applicator ll wetted with theelectrolyte from that in the receptacle 10, serves to apply theelectrolyte to the conductive surfaces that are to be plated.

For the purpose of this invention, I prefer to use a gold platingsolution which is slightly on the alkaline side, that is non-corrosive,and that consists of a solution of sodium gold cyanide, or of potassiumgold cyanide, or of a mixture of sodium gold cyanide and of po tassiumgold cyanide, equivalent to ten grams of sodium gold cyanide (Au Na(CN),) in 16 fluid ounces of water, but as little as 2.5 grams of sodiumgold cyanide per 16 fluid ounces of water can be used satisfactorily.Also, there may be an excess of the selected alkali metal gold cyanidein the gold plating bath over that which can be completely dissolvedtherein, since solution will then take place as gold is plated out inusing the bath.

Instead of sodium gold cyanide, a molecularly equivalent weight ofpotassium or other alkali metal gold cyanide can be used but theresulting electrolyte should contain substantially no free cyanide andno excess of alkali metal cyanide over that bound to or held by the goldcyanide in combination therewith, which combination of gold and alkalimetal cyanide is referred to herein as an alkali metal gold cyanidemoiety. A solution of 10 grams per 16 fluid ounces of water of sodiumgold cyanide and of 3 grams of potassium gold cyanide is most preferred.

The following are analyses of satisfactory sodium and potassium goldcyanide moieties, respectively, expressed in percentages by weight:

Percent by weight 1 Potassium cyanide (K CN).

Electrolytic gold cyanide solutions made with either the sodium orpotassium or other alkali metal cyanide or mixtures of sodium andpotassium gold cyanides should not contain any free or uncombinedcyanides, or any excess of an alkali metal cyanide, since free alkalimetal cyanides cause displacement plating, which is poorly adherent tothe underlying stratum of conductive metal and which is likely to resultin the giving off of fumes that are corrosive and noxious. Wettingagents are not usually needed, but anionic wetting agents can beincluded in the alkali metal gold cyanide solutions if desired.

The avoidance of these undesirable results and the portability of theequipment make my method and electrolyte most suitable for gold platingcontacts or other electrical connections in confined spaces, such asthose in astronautical vehicles, space ship capsules and the like.

The electroplating system of the present invention includes agoldsurfaced electrode'lla in the applicator assembly 11. Said electrode11a can suitably be formed of a conductive, relatively rigid strip ofmetal, such as nickel to give the desired rigidity, with a strip ofrelatively pure gold welded to the working surface of the electrode. Ahandle 12 is secured to the electrode 11a to permit the operator tomanipulate the electrode along any preselected area of the surface to becoated. The electrode 11a is encased in an absorbent sleeve 13 of annon-conductive woven or non-woven fabric .such as cotton,polyacrylonitrile, glass fibers, or similar in ert, porous,fluid-absorbent and retaining material. It is thisabsorbent but inertand non-conductive fabric 13 that prevents direct contact between theanode 1 lo and the surface area 16 to be plated and thus prevents ashort in the plating circuit. The fabric 13, during the platingoperation, is manually held against and rubbed over the surface area 16,with the result that the plating circuit between the gold surfacedelectrode 1 1a and the surface area 16 to be plated is very short,indeed.

' direct current.

The invention as shown in the drawings contemplates providing a goldplating on a laminated printed circuit board 15 having conductive areas16 composed of copper, nickel or the like disposed in a prearrangedpattern on the surface. Contacts 16a are disposed at one edge of theboard for connecting the board electrically with other circuits. Inorder to insure electrical continuity during plating to all of thecontacts 16a on the board, a shorting strip 17 (FIG. 3) composed of anelectrically conductive metal is laid across the surface of the board sothat there is an electrically continuous path through all of the circuitelements 16a at the time electrodeposition takes place. Such strip, asshown in FIG. 3, conveniently consists of a strip of aluminum foil 17asecured to a strip of pressure-sensitive tape 17b with one edge of thefoil flush with an edge of the tape. Alternatively, contact of smallisolated areas on the printed circuit board can be made by connectingthe electrical lead from the power source to a pointed piece ofelectricallyconductive metal thatcan be held r against the edge of thecontact area by hand.

Prior to starting my electroplating process, the area to be platedshould preferably be polished with steel wool to remove any inert matteror burrs or other foreign material. After wiping the surface clean, aclip 18a can be fastened to the shorting strip 17 for providingelectrical current to all the contacts 16a simultaneously.

An electrical conductor 19 connected by a' clip 18 to the electrode 11and an electrical conductor 20 connected to the clip 18a are secured tothe proper terminals of a source of unidirectional current, such as a DCgenerator 22, batteries, a rectifier, or other source of In normaloperation, the voltage from the generator to other source 22 will beadjusted to about 8 to 16 volts, or broadly in the range from 3 to 16volts. With the electrode Hz: the anode and the contacts 16a to beplated the cathode, and with the sleeve 13 thoroughly wetted with theelectrolyte solution, the sleeve is rubbed against the areas to beplated while passing between the anode and cathode a unidirectionalcurrent having a current density equivalent to about 100 to 750 amps/sq.ft. or higher, but usually averaging from 400 to 700 amps/sq. ft. toeffect the plating out of gold over the contact areas 16a. v

Instead of rubbing the gold electrolyte solution onto the surface areasto be plated by means of the gold applicator, but without current, priorto'actual plating, the final cleaning and activating-operation can be.effected'to ensure satisfactory adhesion of the plating by temporarilyreversing the current and applying 1 to volts of reverse current persquare inch for 2 or 3 seconds, and then turning the switch to platingposition, at a voltage of 8 to 12 volts and proceeding with the platingin the manner already described. Plating is continued if it has a darkor brownish superficial appearance, until the desired thickness ofplating has been obtained. This self-indicating appearance assures thata properly conductive, firmly adherent gold plated contact has beenattained. Normally, reaching such an end-point takes place immediatelyupon energizing the plating circuit. Desired thickness of gold platingcan be accomplished by the continued passage of a plating current andcontinued application or a reapplication of the electrolyte by rubbingthe electrolyte-saturated sleeve of the gold applicator assembly overthe already plated surface. After the plating has been completed thework is rinsed with plain warm or cold, and wiped.

The improved method of my invention is applicable to plating gold orcopper, brass and/or nickel surfaces provided the surfaces have beenthoroughly cleaned and polished in the manner earilier described herein.

water,

' After being gold plated, the work should be rinsed with plain water,either hot or cold, and wiped dry. If a' bright finish is desired, it ismerely necessary to polish the dark-appearing gold plating lightly witha 3/0 or 4/0 steel wool and wipe the surface clean with a clean,lintfree cloth. There is, however, no advantage except in appearance inthis polishing step, since the darkappearing gold electrodeposits formedby properly carrying out the gold plating are just as suitable and justas efficient as those that have been polished.

On the other'hand, if during the electroplating step, the goldelectrodeposit remains or becomes clear gold color, like the goldsurface on the anode of the applicator assembly, or changes from asuperficial dark appearance to a clear gold color, that means that theplatgold plating to the work even when carried out by'inexperiencedoperators. Theprocess of my invention makes possible the safe repair andmaintenance of printed circuits and other electronic contacts inconfined spaces of missiles, aerospace equipment, wave guides and otherareaswhich are difficultly accessible. Neither corrosive fumes norcorrosive liquids are generated or caused to bepresent during theoperation of the herein-described method The process of the inventioncan also be used in repair and maintenance work to fill in areas wherethe previously applied coatings have become depleted.

In FIG. 2, certain of the contacts 16a, indicated by the letter B, areshown as having bright gold-colored surfaces, while others of saidcontacts, indicated by the letter D have a superficial brownish-coloredappearance. The B-designatedcontacts, exhibiting a cleargold color, arenot of satisfactory quality or thickness if such clear gold colordevelops and persists asthe direct and inherent result of the platingoperations without any polishing of the surfaces of the'goldplate-having taken place.

On the other hand, if the surfaces of the contacts 16a become and staybrownish-coloreddu'ring plating, like those designated by the letter D,the built-up gold contacts are then of such high quality as to meetmilitary specifications, such as MIL-045204, and stay firmly adherent tothe under structure even when flexed. It of the desired thickness, say,at least 0.00005 inches, the contacts are satisfactory in the as-platedstate for use without further processing, but for appearances sake thesurfaces can be lightly polished to present a clear gold surface. Thedescription of the D contacts as dark", or brownish is in contrast tothe clear yellow color. characteristic of gold, and is very marked andeasily observable, as is alsothe contrast between the superficialbrownish coloration of the'D contacts and the characteristically goldappearance of the gold on the surface 11a of the applicator electrode 1l. The use of my non-displacement type of gold electrolye combinationwith the relatively close proximity, usually about 1/16 inch, of thesurface of the gold anode to the surface area 16a to be plated, insuresa satisfactory gold plating of said surface area with a superficialbrownishcolored gold layer, provided the plating circuit is properlycompleted to include all of the areas to be plated. Such completion ofthe electrical circuit through the assembled printed circuits andcomponent assemblies, when including many discrete contact areas, cansometimes be best accomplished preparatory to, or during actual plating,by pressing a flexible mass of 1/10 or 2/0 steel wool against all of thecontact areas 16a simultaneously by means of an electrically conductivestrip or bar of copper or brass. Such a strip or bar is used in lieu ofthe shorting strip 17 of FIG. 3 to short the contact areas 16a.

In the practice of the method of my invention, so long as the goldplating is of a brownish color, the plating is satisfactory and can becontinued until the desired thickness of the plating has been reached.Since under these conditions the rate of deposition remains uniform, agiven thickness of electrodeposit will be obtained in a certain lengthof plating time. Thickness of the plating can thus be calculateddirectly from the length of time of plating and the known rate ofelectrod'eposition of a unit thickness of gold.

I claim as my invention:

l. The method of electrodepositing gold on an area of an electricallyconductive metal surface to provide a firmly adherent high quality goldplate over said area,

which comprises establishing a plating current between said surface anda manually guided pure gold anode through a gold electrolyte solutioncarried by a nonconductive, absorbent fabric between said anode and saidsurface, effecting wetting and rubbing contact between said absorbentmaterial and said surface as the cathode to effect gold plating of saidarea,

said anode having a backing of a more rigid metal than gold incapable ofcodepositing with the gold from said gold electrolyte solution,

said electrolyte solution consisting essentially of an aqueous solutionof an alkali metal gold cyanide moiety containing substantially no freecyanide and substantially no free alkali metal cyanide not bound to thegold present in said gold cyanide moiety, and

continuing said gold plating to produce a gold plate having asuperficial brownish coloration in contrast to the clear gold colorationcharacteristic of gold,

. said brownish color persisting during such continued plating, and

stopping said plating when the desired thickness of said gold plate hasbeen reached.

2. The method as defined by claim 1, wherein said plating circuitcarries a current density of at least about amps/sq. ft. and is at avoltage of from about 8 to 16 volts.

3. The method as defined by claim 2, wherein, the current density isfrom 400 to 700 amps/sq. ft. and higher.

4. The method as defined by claim 1, wherein said surface area to begold plated includes many discrete area portions, and all of said areaportions are simultaneously electrically connected during plating by aflexible mass of steel wool held thereagainst and contacting said areas.

5. A non-displacement type of gold plating electrolyte solutionconsisting essentially of an aqueous solution of an alkali metal goldcyanide moiety containing substantially no free cyanide andsubstantially no free alkali metal cyanide not bound to the gold presentin a concentration molecularly equivalent to from about 2.5 grams toabout 10 grams of sodium gold cyanide, (Au Na (CN),), per 16 fluidounces, said aqueous solution containing substantially no free cyanideand substantially no free alkali metal cyanide, and substantially all ofthe cyanide present including the alkali metal cyanide being bound tosaid gold.

6. The method of electrodepositing gold on an area of an electricallyconductive metal surface to provide a firmly adherent high quality goldplate over said area, which comprises establishing a plating current ofat least 400 amps/sq.

ft. density between said surface and a manually guided, effectively puregold anode through a gold electrolyte solution carried by anonconductive, absorbent fabric enveloping said anode and maintained inpressure contact with both said anode and said surface, effectingwetting and rubbing contact between said absorbent fabric and saidsurface with said surface constituting the cathode to effect goldplating of said area,

said anode having a backing of a more rigid metal than gold incapable ofcodepositing with the gold from said gold electrolyte solution; saidelectrolyte solution consisting essentially of an aqueous solution of analkali metal gold cyanide moiety containing substantially no freecyanide and substantially no free alkali metal cyanide not bound to thegold present in said gold cyanide moiety, and containing no additionagent of any kind, and continuing said gold plating to produce a goldplate having a superficial brownish coloration in contrast to the cleargold coloration characteristic of gold,

said brownish coloration persisting during such continued plating, and

stopping said plating whenthe desired thickness of said gold plate hasbeen reached.

2. The method as defined by claim 1, wherein said plating circuitcarries a current density of at least about 100 amps./sq. ft. and is ata voltage of from about 8 to 16 volts.
 3. The method as defined by claim2, wherein, the current density is from 400 to 700 amps./sq. ft. andhigher.
 4. The method as defined by claim 1, wherein said surface areato be gold plated includes many discrete area portions, and all of saidarea portions are simultaneously electrically connected during platingby a flexible mass of steel wool held thereagainst and contacting saidareas.
 5. A non-displacement type of gold plating electrolyte solutionconsisting essentially of an aqueous solution of an alkali metal goldcyanide moiety containing substantially no free cyanide andsubstantially no free alkali metal cyanide not bound to the gold presentin a concentration molecularly equivalent to from about 2.5 grams toabout 10 grams of sodium gold cyanide, (Au Na (CN)2), per 16 fluidounces, said aqueous solution containing substantially no free cyanideand substantially no free alkali metal cyanide, and substantially all ofthe cyanide present including the alkali metal cyanide being bound tosaid gold.
 6. The method of electrodepositing gold on an area of anelectrically conductive metal surface to provide a firmly adherent highquality gold plate over said area, which comprises establishing aplating current of at least 400 amps./sq. ft. density between saidsurface and a manually guided, effectively pure gold anode through agold electrolyte solution carried by a nonconductive, absorbent fabricenveloping said anode and maintained in pressure contact with both saidanode and said surface, effecting wetting and rubbing contact betweensaid absorbent fabric and said surface with said surface constitutingthe cathode to effect gold plating of said area, said anode having abacking of a more rigid metal than gold incapable of codepositing withthe gold from said gold electrolyte solution; said electrolyte solutionconsisting essentially of an aqueous solution of an alkali metal goldcyanide moiety containing substantially no free cyanide andsubstantially no free alkali metal cyanide not bound to the gold presentin said gold cyanide moiety, and containing no addition agent of anykind, and continuing said gold plating to produce a gold plate having asuperficial brownish coloration in contrast to the clear gold colorationcharacteristic of gold, said brownish coloration persisting during suchcontinued plating, and stopping said plating when the desired thicknessof said gold plate has been reached.